1. Field of the Invention
The present invention relates to antennas for wireless communication. The invention particularly relates to a composite antenna device which combines a plurality of antennas into one piece to simultaneously support wireless communications involving different standards.
2. Description of the Related Art
Portable wireless terminals such as a cellular phone, laptop, netbook, sensor network, ultra-mobile personal computer (UMPC), personal navigation device (PND), etc., have been increasingly made smaller, lighter, and thinner with more sophisticated functions, which has increased demands on internal parts to be made smaller. In addition, since wireless communication is crowded with different standards, antennas are often required to support a plurality of frequency bands and different polarized waves.
JP-A 2004-015096 discloses a composite antenna device that responds to a plurality of frequency bands yet is not affected by mutual interference between reception units. The composite antenna device stated in JP-A 2004-015096 has a first antenna unit for communication equipment being responsive to a wave in a first radio frequency band; and a second antenna unit for communication equipment being responsive to a wave in a radio frequency band that is different from the first radio frequency band, in which a trap circuit is provided to the second antenna unit for eliminating interference caused by the wave in the radio frequency band of the first antenna unit.
FIG. 1 is a structural diagram of the trap circuit provided to the second antenna unit in JP-A 2004-015096. As shown in FIG. 1, a trap circuit 102 responsive to the first radio frequency band and a trap circuit 103 responsive to another radio frequency band are provided between an antenna 100 and an LNA (low noise amplifier) input circuit 101.
JP-A 2004-187148 discloses a composite antenna device, in which a plurality of antenna elements having different target frequency bands are more compactly arranged while preventing interference between individual elements. FIG. 2 is a schematic illustration showing a perspective view of a structural appearance of the composite antenna device according to an embodiment of JP-A 2004-187148. In the composite antenna device stated in JP-A 2004-187148, as shown in FIG. 2, an antenna element 210 for GPS (global positioning system), in which an element pattern 212 is printed on a rectangular dielectric plate 211, and an antenna element 220 for ETC (electronic toll collection system), in which an element pattern 222 is printed on a rectangular dielectric plate 221, are arranged in parallel on an antenna substrate 201. In addition, the four sides and the bottom surface of the dielectric plate 221, on which the element pattern 222 for the ETC antenna is formed, is surrounded by a grounding plate 223.
JP-A 2004-015096 states that electric signals in unnecessary radio frequency bands can be shut out by providing the trap circuits 102 and 103 for signals received by the antenna 100, which prevents the device from being affected by mutual interference between the reception units responsive to a plurality of radio frequency bands. However, no concrete explanation is given with regard to the reduction of interference between the first and the second antenna units, which is the underlying cause of the interference, and a challenge still remains which is to reduce interference noise in a target frequency band, generated at a reception stage.
Furthermore, JP-A 2004-187148 states that, by taking at least one of the plurality of antenna elements, 210 and 220, and surrounding the sides and the bottom of the dielectric plate of the selected element pattern with the grounding plate 223, mutual interference between the antenna elements can be reduced in spite of the close arrangement. This technique seems effective in reducing interference when the antenna elements are patch antennas like those described in JP-A 2004-187148; however, when the antenna elements have different structures (for example, slot antennas, etc.), the technique may be difficult to apply or may not be able to reduce interference sufficiently.